1. Field of the Invention
The present invention relates to a safety coupling designed to transfer torque between two machine components sharing the same axis.
2. Description of the Related Art
An exemplification of such a safety coupling is published in the German paper DE-OS-29-23-902. The safety coupling serves to transfer torque between two machine components sharing the same axis, while preventing unacceptably high torque increases. The safety coupling includes at least one thin-walled sleeve, forming a wall of a ring-shaped chamber extending in axial direction. The ring-shaped chamber can be pressurized with a medium in order to elastically deform the sleeve in radial direction, causing it to jam against the surface of an element onto which the coupling is mounted. Adjacent to the ring-shaped chamber are drillings, which are a part of a safety device or coupling relief device. As a result of the relative motion between the surfaces and the actions of the relief mechanism, the pressurized medium residing in the ring-shaped chamber can escape through the drillings, thus lowering the pressure inside the chamber.
To transfer a certain torque level, a certain surface pressure is required. For that purpose, oil is being pumped into the chamber, which is needed to deform the respective machine components relative to one another. In this way, the coupling is adjusted to the desired torque capacity. If, during an overload condition, this torque is exceeded, the coupling slips. The maximum torque level that can be transmitted is lowered because the effective, static friction coefficient transitions into the sliding friction coefficient. There is a relative motion in circumferential direction between the individual elements of the two machine components, which are jammed relative to one another. A shear disk mounted on one machine component shears off a shear valve, opening the connection to the ring-shaped chamber of the coupling. After shearing off the shearing valve (or valves), the pressurized oil can freely expand and the torque to be transmitted reduces to zero within a few milliseconds.
The disadvantage of these types of safety couplings centers around the fact that these couplings respond only when there is a relative motion between the surfaces which are jammed against one another (one of which is formed by the coupling body), or upon or a certain drivetrain-related torsional motion. This can cause permanent, irreparable damage in the drivetrain, which is reflected in an increase in the cost associated with the re-setting of the coupling to its original state as it operated prior to its activation. Furthermore, the relative motion or torsional deformations are directly linked to each other. Thus, the placement of the coupling is restricted to those areas, in which high torque levels should be avoided, thereby limiting the scope of the application of this coupling from the very beginning.